Do not list any directories as keys in the 'salt-call -L' output.

**salt-key will list directories as if they were minion key entries**

This is more a cosmetic bug. When using subversion to manage the ```/etc/salt``` directory, it creates ```.svn``` directories underneath the ```/etc/salt/pki/master/minions```, ```/etc/salt/pki/master/minions/minions_pre``` and ```/etc/salt/pki/master/minions/minions_rejected``` directories. These directories (or any manually created subdirectories for that matter) show up in the ```salt-key -L``` output as if they were accepted, pending and rejected keys, respectively:
```text
~ $ PYTHONPATH=. ./scripts/salt-key -L

Accepted Keys:
.svn
minion
Unaccepted Keys:
.svn
some_directory
Rejected Keys:
.svn
```

If I "accept" the ```some_directory``` key in the example above, this directory gets moved from the unaccepted keys to the accepted keys, but the operation makes no sense.

The list is generated in ```salt.key.Key```'s ```list_keys()``` method by simply using an ```os.listdir()``` on the three minion key directories (approved, pending and rejected) and adding anything it returns. A simple extra check should make this a bit prettier, since (if I understand the code in ```Key``` correctly) directories as keys are not really supported.